Gas supply pipes are employed in which, after a passivated film, the surface of which comprises solely chromium oxide does not contain iron oxides! is formed on the inner surface of pipes comprising an austenitic stainless steel such as SUS 316 or the like, the pipes are welded together.
However, when such gas supply pipes are employed, the life span of the pipes is extremely short. Accordingly, it is necessary to replace the pipes at extremely short intervals, and this involves a great deal of work and expense. Furthermore, as a result of such replacement, there is a tendency for the degree of cleanliness within the clean room to decline.
Upon investigating the causes of the short lifespan of such pipes, the present inventors have discovered that passivated films containing iron oxides are formed at the welded parts, and corrosion is generated at these parts.